• 4,5 +
• 7,8 -

Cheap passive PoE injectors probably use pairs (4,5) and (7,8) exclusively for power, and disconnect them from data, so probably will only work at 100 Mbps.

I bought some cheap PoE injector/splitters from ebay. They claim gigabit speed, and a quick test shows they negotiate at that.

Pins 1,2,3,6 are directly connected through. Pins 4,5,7,8 seems to be connected with a serial DC blocking cap of what my multimeter says is ~93nF.

Cheap component testers says:

• 0.41mH power to combined side pin 4.
• 0.73mH between combine side pins 4 and 5.

It seems to be a bias T! I was expecting transformers with central tap, as is used on the ends normally.

I thought 12V would be enough for low power stuff / short wires, but with the Pi in a box even 10 meters is pushing it at 12V.

The Pi crashed unpleasantly, causing file system corruption, when running with 12V into a high resistance cable of 2.5R per wire.

I will install box 0 with a 20V supply I found.

pi draws up to 3a in normal non PD mode at 5V - 15W.
Heltec board up to about 2.5W.
Other bits maybe max 0.5W.
18W needed.
18W @ 12V about 1.5A + conversion loss, say 1.7A.

12V supply is 3A rated.
Converter is fine at 3A (tested).

CAT5e I have is 24 AWG.
24AWG copper: 88 Ohm / km
For 10m: 0.88 Ohm
For a pair: 0.44 Ohm / 10m.
For a pair including return: 0.88 Ohm.

It's more though, because of twisting, copper is longer than cable length.
Say 1 Ohm per 10 meters. It might be more actually.

Max R (including both sides) 1.75 Ohm.

20m is therefore too long for this cable / voltage.

With 20V, in max acceptable R is about 4.48R.
So then 40m is ok.

Apparently Cat 6 is usually AWG 23.

Four pins, each connects to centre tap of each pair's transformer.

• + (1,2)
• - (3,6)

Only needs two pairs for 100 Mbps and power.